One of the most economic and commonly used methods to manufacture polymers is gas phase polymerization. A conventional gas phase fluidized bed reactor used in polymerizing olefins and/or diolefins contains a fluidized dense-phase bed (i.e., the mixture of reaction gas and polymer (resin) particles) and a freeboard above the dense-phase surface (bed level). The freeboard contains mainly gas and a small amount of particles, especially the fine particles (fines). The dense-phase bed is usually maintained in a cylindrical straight section of the reactor. Above the straight section, there is a section having a larger diameter, the so called expanded section, to reduce the gas velocity for the purpose of reducing the amount of fines carried out of the reactor to other parts of the reaction system. The expanded section connects with the straight section by its tapered conical section. The freeboard is usually located at the expanded section. If the bed level is lower than the top of the straight section, the upper portion of the straight section also becomes a part of the freeboard.
During reactor operation, fines present in the freeboard will either be carried away by the gas leaving the reactor or fall back into the dense-phase. However, some fines may attach onto the interior wall of the reactor system, particularly in the freeboard portion, and accumulate to form so-called sheets, i.e., layers of agglomerated, melted or half-melted, resin and catalyst particles. Sheets can adversely affect properties of the polymer product. When sheets become heavy, they can fall off the walls and plug the product discharge system or clog the distributor plate. Small pieces of sheets can be discharged together with the bulk resin particles and contribute to product quality problems by increasing the gel level of end-use products such as plastic containers and films. Sheeting and fines accumulation are collectively referred to as solid particle build-up.
Conventionally, to prevent sheeting from affecting these and other parts of the reaction system, as well as the final polymer product, the reactors are shutdown periodically and the walls are cleaned. When a reactor is down for cleaning, large amounts of operation time are lost, in addition to the cost of the cleaning itself. Thus, a method to continuously clean the interior wall in the reactor freeboard and other parts of the reaction system can provide savings of time and money.
In U.S. Pat. No. 5,461,123, the reactor wall is protected from particle build-up by sound waves. According to that invention, sound waves are introduced into the reactor to loosen particles attached on the wall. Then the loosened particles can be carried away from the wall by gravity or drag forces.
However, it has been discovered that the performance of the sound waves relies on several parameters, e.g., number of sonic nozzles, location of sonic nozzles, orientation of sonic nozzles, sound pressure level, sound wave frequency, sonic operation mode (including duration and interval), length of sonic tubes, diameter of sonic tubes, sonic tube insertion length, etc. The proper selection of these design and operating parameters is necessary to achieve optimal protection of the reactor freeboard, otherwise the effectiveness of the sound waves can be diminished or even eliminated.
It will be desirable to provide an improved gas phase polyolefin polymerization process using sonic devices with optimum design and operation. It is the object of this invention to provide methods to optimize the design and operation of the sonic cleaning device(s).